This invention relates to methods of producing transition joints between ferritic steel components and to transition joints so produced.
When joining metal pieces together by welding or fusion, three metallurgical zones are created: a fusion zone; a heat affected zone and an unaffected zone.
The fusion zone presents a cast structure that is only partially refined by successive welding runs and thermal treatments, making inspection of the weld difficult to achieve except by radiography.
The thermal history associated with the welding operation gives rise to an area of coarse grains (called the heat affected zone), lying between the fusion zone and the bulk material. This grain size variation creates an interface which makes the inspection of the weld by ultrasonic methods very difficult resulting in the need to use the inferior and time consuming process of radiography.
Many applications call for the joining of dissimilar materials. For example, in the power generation industry there is a need to join together components produced in varying grades of ferritic steel. Most of the jointing methods employed utilize a transition joint where the difficult weld between the two dissimilar metals is produced in a controlled environment, leaving the two metal fusion welds to be carried out in-situ.